The invention relates to an apparatus for heat treatment of material to be worked on, such as cast strips and billets, as well as ingots, rods, tubes and the like, especially of aluminum or magnesium alloys. The heat treatment is of the general type in which the material is first pre-heated, and thereafter is held at a desired heat treatment temperature.
Cast strips, billets and extrusion and rolling products, are customarily subjected to a heat treatment in order to homogenize, heterogenize, or otherwise heat treat the material. For example, continuously cast billets of aluminum alloys are first pre-heated after the casting, then finally annealed at temperatures between 500.degree. and 620.degree. C., and thereafter cooled. During this treatment, the billets receive the structure desired for further working, such as for example extrusion or rolling. An example of such methods is found in U.S. Pat. No. 2,802,657 (Nesbitt).
The material is customarily pre-heated with circulated hot gas, flue gas or with circulated hot air. As a result of the comparatively low temperature of such a source of heat or "heater," the pre-heating step takes a very long time.
If the material is to be passed through the apparatus in a continuous manner or in a flow operation, one normally tries to transport the material at an equal and constant speed through the pre-heating zone and subsequently through the holding heat treatment zone in the furnace. If the pre-heating is of long duration, then the pre-heating zone must be disproportionately long with respect to the holding zone or the material, and upon entry into the holding zone, will not attain the proper annealing temperature.
In using prior art apparatus, in order to achieve different annealing temperatures, the temperature of the hot gas in the pre-heating area or zone, and in the holding phase area or zone must be finely controllable. This is normally very difficult and a change in the temperature of the hot gas is usually only possible wihin narrow limits. Additionally, as was noted above, in a flow-through operation, the material must normally be transported with equal speed through the pre-heating zone and the holding zone.
As a result of the above, there is imposed upon normal operations a restricted flexibility in treatment of materials, especially a restricted adaptability to the changing of conditions under which the material is treated.
An additional serious drawback in known devices is that during heat treatment, which eventually also comprises subsequent cooling, the material suffers uneven deformations which may result in distortion or bending of the material.